1. Field of the Invention
The present invention relates to utilization of a compound having a seven-membered ring imide structure as a photoradical generator, a photosensitive resin composition containing the photoradical generator and a product produced using the photosensitive resin composition.
The invention relates, particularly, to a photoradical generator which has high heat resistance and stability, is excellent in compatibility with other components such as of a monomer or the like and solubility to a solvent, is high in transparency, and remains little decomposition material of low molecular weight and unstable unreacted substance after a radical reaction. Also, the invention relates to a photosensitive resin composition which contains the photoradical generator, is high in sensitivity, is excellent in storage stability and is less liable to emit a volatile component and a odor upon light radiation resulting from an initiator, and further relates to an article at least a part of which is formed of the photosensitive resin composition, and which has little low molecular or unstable residue derived from an initiator remained, and has high heat resistance, stability and quality.
2. Description of the Related Art
Photosensitive resins which are cured or changed in solubility by irradiation of radial rays such as ultraviolet rays are generally classified into two categories including a type (positive type) in which an exposed portion is highly soluble and a type (negative type) in which an unexposed portion is highly soluble. In the case of the negative type, the photosensitive resin remains on a substrate and often becomes a part of a product as a functional layer because the photosensitive resin itself is cured and becomes insoluble by exposure.
Although the negative type photosensitive resin has been used for, for example, paints, printing inks, overcoat layers, adhesives, printing master plates or the like, it has been recently used in wide applications ranging to products such as solder resists for wire protection in printed wiring boards, layer insulation films and resists for forming pixels in color filters, antireflection films, hologram or the like.
As one of the negative type photosensitive resins which are frequently used in general, there are resin compositions containing a compound having one or more ethylenic unsaturated bonds, a photoradical initiator generating a radical by irradiation with light and, as required, a high molecular compound providing developing ability, softness to a coating layer or the like, an inorganic filler, pigments or the like. When radial rays are applied to this composition, the compounds having an ethylenic unsaturated bond are bonded to each other by a radical reaction and cured as a macromolecule. In this curing reaction, a three-dimensional network structure is developed by a crosslinking reaction, improving the hardness, strength, adhesiveness, resistance to solvent and heat resistance of the resulting cured product.
The photoradical initiators are roughly classified into a self cleavage type and a hydrogen-drawing type. In the case of the former type, it absorbs light (radial rays) having a specific wavelength, the bond at a part corresponding to that wavelength is cut, radicals are generated in each part cut at this time and a radical reaction starts from these parts. In the case of the latter type, when it absorbs a radial ray having a specific wave length and put into an excited state, it draws hydrogen from surrounding hydrogen donors. At this time, radicals are generated from each of the drawing one and the drawn one.
Generally, the self-cleavage type has good sensitivity and high radical-generating efficiency, but is unstable to heat, giving rise to the problems concerning the heat resistance, stability and preserving stability of a photosensitive resin composition containing this self-cleavage type. In the case of the hydrogen-drawing type, on the other hand, the hydrogen donor must exist in the vicinity of the excited initiator and the radical-generating efficiency is decided by the magnitude of an energy barrier when drawing hydrogen. Therefore, this type has relatively low sensitivity, but a resin composition containing this type exhibits high stability and preserving stability because no radical is generated unless it is put in an excited state and draws hydrogen.
In the solder resist used for surface coating in a print board, an organic pigment and filler are compounded to provide heat resistance and flame resistance. Also, in a resist used for forming pixels of a color filter, a pigment for color indication is compounded. Because these pigments are components absorbing light, a self-cleavage type photoradical initiator is principally used to raise the sensitivity of a photosensitive resin and also the photoradical initiator is compounded in a large amount allowing for a portion which will not fully used in a radical reaction. Here, the portion which is not fully used in a radical reaction includes an unreacted initiator which has not been cleft even by irradiation with light and an initiator whose activity is lost by prohibiting its access to material to be caused reaction because of a reaction in a solid phase even if it is radicalized by cleavage.
A large amount of residues originated from an initiator exists in the cured product after exposure. Among these residues, a photoradical initiator which has not been cleft still keeps reactivity even after exposure and therefore denatures a product. Also, a photoradical initiator which has not been cleft and a decomposed material which has been cleft but not consumed in a radical reaction and deactivated is not bonded to a crosslinking structure of a matrix and exists as an independent component in a product, which impairs the quality of a layer. For this, if a residue originated from an initiator is left as it is, there is a problem that it causes deteriorated light resistance, coloring, fading, peeling of a coating layer, the occurrence of cracks or the like, which is a cause of a reduction in the reliability of a final product such as a layer insulation film and solder resist in electronic parts and a resist used for forming pixels in a color filter.
The self-cleavage type photoradical initiator has a strong sublimation tendency and is decomposed by heat. It can be therefore removed from a product by post-baking after exposure and developing at a temperature higher than hundred and several tens degree. However, a large amount of a sublimated material originated from an initiator adheres to the inside of a heater and falls on a product obtained by curing during post-baking, causing product defects, posing a serious problem. Also, a decomposed material of an initiator or the like is involved in the atmosphere around the heater, posing a problem from the viewpoint of operational safety.
It is possible to remove more residues originated from a radical initiator by changing a post-baking condition to a condition of a higher-temperature and longer-operation time. However, it is difficult to remove the residues completely because of volatilization from a solid. If the condition is made stricter to remove many more impurities originated from a radical initiator, this condition rather causes product defects.
In the meantime, the same curing system using radial rays is applied to a resist for processing electronic parts to be used as a peelable layer and a dry film resist or the like. The processing resist is finally peeled off and is not therefore left in a product. However, the processing step such as formation of copper wirings or the like involves such a problem that a residue originated from an initiator is eluted in a chemical solution such as ferric chloride and cupric chloride used for the processing from a resist film, so that the life of the chemical solution is shortened.
Moreover, when a photosensitive resin is used as a wall used for buildings or paint for a protective layer protecting a surface of wall paper, there is a demand for decreasing solvent components or odorous components emitting from whole building material with the view of dealing with sick house syndrome. There is a problem that the use of a highly volatile initiator causes the occurrence of odors even after curing.
From these problems, it has been desired to develop a radical generator and a resin composition which are not volatilized during post-baking and after photo-curing and are substantially free of components which are originated from a radical generator and remain independently in a coating layer.
As measures to solve these problems, ESACURE KIP 150 (trademark, manufactured by Nihon Siber Hegner CO., LTD.) or the like introduces a photoradical-generating part into the side chain of a polymer skeleton. This measure ensures that a photoradical generator has plural radical-generating parts in one molecule. Therefore, if any one part in the molecule is radicalized and bonded to a matrix of the coating layer, an unreacted radical-generating part in the same molecule is bonded to the matrix through the polymer skeleton. For this, the photoradical generator is not volatilized during post-baking and does not move in the coating layer, and deterioration of the reliability of a final product is a little.
In this case, however, the photoradical-generating part introduced into the side chain is a self-cleavage type and is easily decomposed by heating to produce a radical, still posing the same problems concerning, for example, the heat resistance, stability and preserving ability of a photosensitive resin composition containing this product. Although, among the radical-generating parts, the part left on the polymer skeleton after cleavage is bonded to the matrix structure, a part of the decomposed material cleft from the polymer skeleton by a photoradical reaction and post-baking remains independently not bonded to the matrix structure. Therefore, if the decomposed material is left as it is, it adversely affects the physical properties of a coating layer, and it is also difficult to remove the decomposed material perfectly by sublimation even if post-baking is carried out.
There is a proposal as to the use of (meth)acrylates having a maleimide group in each publication of International Publication No. WO98/58912 and Japanese Patent Application Laid-Open No. 2002-3559. These (meth)acrylates react with vinyl ether to generate a radical as an electron acceptor because maleimide absorbs electromagnetic waves. Also, these (meth)acrylates draw hydrogen, whereby a radical can also be generated (RADICAL POLYMERIZATION HANDBOOK, NTS Co., Ltd., 1999, page 312). However, maleimide has an ethylenic double bond. If a monomer having both maleimide group and (meth)acryl group is radical-polymerized, a crosslinking reaction proceeds and the resulting polymer is gelled. For this, according to each publication of International Publication No. WO98/58912 and Japanese Patent Application Laid-Open No. 2002-3559, a substituent such as a cyclohexyl group is introduced into a maleimide group to lower the reactivity of the maleimide group by steric hindrance, whereby the above problem is overcome. However, since the reactivity of the maleimide is lowered on the contrary, there is the problem that the efficiency of initiation of a radical reaction is dropped. Also, because the reaction for forming a maleimide group by reacting an acid anhydride with an amine is carried out by a dehydration reaction, a temperature as high as 100° C. or more is required to react highly efficiently without using a catalyst. If it is intended to introduce an ethylenic unsaturated bond directly when forming the maleimide group, there is a synthesis problem that polymerization of the ethylenic unsaturated bond is caused. Also, although a dehydrating catalyst such as acetic acid anhydride or the like may be used to carry out a dehydration reaction. However, this causes a cost increase and also the subsequent refining process is complicated, giving rise to a synthesis problem in any case.
In the meantime, Kubo et al. report that when N-methyl-1,8-naphthalimide and an aromatic compound such as p-xylene or the like are irradiated with ultraviolet rays in the presence of methanol in an acetonitrile solution, a reaction product of naphthalimide and the aromatic compound is obtained in a high yield (Chemistry Letters, 1999, page 175). There is the description in this report that as its reaction mechanism, naphthalimide which is singlet-excited by ultraviolet rays forms an exciplex with the aromatic compound and then the exciplex generates a radical by drawing hydrogen to form a bond between the naphthalimide and the aromatic compound. However, in this document, there is only a description of a reaction in a solution between naphthalimide and low molecular aromatic compounds in a partial scope.
In light of the aforementioned knowledge, the inventors of the present invention proposed a photoradical polymerization initiator having a group(s) containing naphthalimide structure which is a six-membered ring structure imide group, and a radial sensitive resin composition (Japanese Patent Application No. 2003-88582). The naphthalimide containing group functions as a hydrogen-drawing type photoradical generating part, hence, the naphthalimide does not decompose but generates radical in a photoradical generating process, and does not generate radical in a general heating process. The naphthalimide structure itself has high heat resistance. Also, the naphthalimide structure can be synthesized in a relatively moderate condition. Therefore, a photoradical polymerization having a group containing a naphthalimide structure has high heat resistance, and low-molecular decomposition materials or unreacted products do not remain in a product (particularly, a coating layer after curing) after a radical reaction. Further, the photoradical polymerization initiator having a group containing a naphthalimide structure can be relatively easily produced.
However, the photoradical polymerization initiator having a group containing a naphthalimide structure has not so high compatibility with acrylic polyfunctional monomer generally used for a negative type photosensitive resin composition or solubility to general solvent, thus, there has been a problem that the photoradical polymerization initiator deposits when contained in high concentration in a photosensitive resin composition.